text: Strong light causes the reduction of the plastoquinone pool, which leads to protonation and double reduction (and double protonation) of the Q electron acceptor of Photosystem II. The protonated and double-reduced forms of Q do not function in electron transport. Furthermore, charge recombination reactions in inhibited Photosystem II are expected to lead to the triplet state of the primary donor (P) more probably than same reactions in active PSII. Triplet P may react with oxygen to produce harmful singlet oxygen.